Malaria is one of the most devastating parasitic diseases affecting humans. The Centers for Disease Control (CDC) estimate that over three billion people live in areas at risk of malaria transmission in 106 countries and territories (e.g., parts of Africa, Asia, the Middle East, Central and South America, Hispaniola, and Oceania). The World Health Organization (WHO) and CDC also estimate that in 2010, malaria caused over 200 million clinical episodes 655,000 deaths, with the majority of deaths occurring in Africa. About 86% of the malaria deaths in 2010 occurred in children. On average, 1,500 cases of malaria are reported annually in the United States, and malaria is a major health concern to U.S. military personnel deployed to tropical regions of the world. For example, in August 2003, 28% of the 26th Marine Expeditionary Unit and Joint Task Force briefly deployed to Monrovia, Liberia, were infected with the malaria parasite Plasmodium falciparum. In addition, one 157-man Marine Expeditionary Unit sustained a 44% malaria casualty rate over a 12-day period while stationed at Robert International Airport in Monrovia. In all conflicts during the past century conducted in malaria endemic areas, malaria has been the leading cause of casualties, exceeding enemy-inflicted casualties in its impact on “person-days” lost from duty.
To combat malaria during U.S. military operations, preventive drugs, insect repellants, and barriers have been used with some success, but developing drug resistance by the malaria parasite and insecticide resistance by mosquito vectors has limited the efficacy of these agents. Moreover, the logistical burden and side effects associated with the use of these agents often is associated with high non-compliance rates. Vaccines are the most cost effective and efficient therapeutic interventions for infectious diseases. In this regard, vaccination has the advantage of administration prior to military deployment and likely reduction in non-compliance risks. However, decades of research and development directed to a malaria vaccine have not proven successful. Recent efforts have focused on developing vaccines against several specific malaria genes and delivery vector systems including adenovirus, poxvirus, and plasmids. The current status of malaria vaccine development and clinical trials is reviewed in, for example, Vaughan et al., Curr. Opin., Immunol., 24(3): 324-331 (2012); Schwartz et al., Malaria Journal, 11: 11 (2012); Tyagi et al., J. Control Release, 162(1): 242-254 (2012); Graves and Gelband, Cochrane Database Syst. Rev., 1: CD000129 (2003); Moore et al., Lancet Infect. Dis., 2: 737-743 (2002); Carvalho et al., Scand. Immunol., 56: 327-343 (2002); Moorthy and Hill, Br. Med. Bull., 62: 59-72 (2002); Greenwood and Alonso, Chem. Immunol., 80: 366-395 (2002); and Richie and Saul, Nature, 415: 694-701 (2002).
An unprecedented quantity of genomic data has emerged from the sequencing and functional genomic analysis of many disease-causing organisms, including malaria. Indeed, it has been determined that the parasite Plasmodium falciparum encodes an estimated 5,268 putative proteins (see Gardner et al., Nature, 419: 498-511 (2002)). This genetic information can be exploited for the systematic discovery of novel antigens for vaccine development. In the past, target antigens for genetic vaccines have been identified based mainly on their abundance in the pathogen of interest and their susceptibility to neutralization by antibodies generated in infected individuals and animal models. This approach has failed to yield effective vaccines against many of the most devastating infectious diseases.
With regard to malaria, less than 5% of the Plasmodium falciparum genome is represented by antigens currently in clinical development. However, a number of potential vaccine candidates targeted against pre-erythrocytic, erythrocytic and sexual stages of P. falciparum are under various stages of clinical development (see, e.g., Crompton et al., J. Clin. Invest., 120: 4168-4178 (2010)). The RTS,S vaccine is the most clinically advance malaria vaccine. RTS,S is a pre-erythrocytic stage vaccine based on the P. falciparum circumsporozoite protein (CSP), and provides protective efficacy in phase II clinical trials of 30-50% against pathogen challenge (see, e.g., Cesares et al., Vaccine, 28: 4880-4894 (2010); Stoute et al., N. Engl. J. Med., 336: 86-91 (1997); Kester et al., Vaccine, 26: 2191-2202 (2008); Kester et al., J. Infect. Dis., 200: 337-346 (2009); Kester et al., Vaccine, 25: 5359-5366 (2007); and Zeeshan et al., PLoS ONE, 7(8): e43430 (2012)). Initial results of phase III clinical trials show that the RTS,S vaccine provides protective efficacies of 56% and 47% against clinical and severe malaria, respectively, in African children age 5 to 17 months (see, e.g., Agnandji et al., N. Engl. J. Med., 365: 1863-1875 (2011)). The protection afforded by this protein-based vaccine, however, is short lived (3-8 weeks).
Other recent efforts at developing a malaria vaccine have focused on several specific genes and their delivery using various different vector systems including adenovirus, poxvirus, and plasmid DNA. It is not apparent, however, whether these recombinant vaccines are effective against malaria, or if they encode the most potent protective antigens. It is clear that protective antigens do exist for the malaria pathogen Plasmodium falciparum, as evidenced by the ability of irradiated sporozoites to induce cellular immune responses in human subjects and robust sterile protection against parasite challenge (see, e.g., Nussenzweig and Nussenzweig, Adv. Immunol., 45: 283-334 (1989), and Hoffman et al., J. Infect. Dis., 185: 1155-1164 (2002)).
Thus, there remains a need for compositions containing improved antigens that induce potent protective immunity against challenge with malaria-causing parasites. The invention provides such a composition. This and other advantages of the invention will become apparent from the detailed description provided herein.